One approach to the problem of providing digital data communication service to an increasing number of customer stations in a time division switching system relies on the low data rates required by many of those stations. The approach, referred to as subrate switching, has groups of like data rate stations share channels of the switching system. For example, in a system having 128 channels per 125-microsecond frame, a group of twenty stations having a data rate of one data word per twenty frames can share one channel of the system.
Time division switching systems generally function in repetitive time slots of fixed duration. During each time slot a data word from an originating station is switched to the destination station defined by a control word associated with that time slot. The control words which control the switching system are typically stored by the switching system as at least one list which is accessed in a predetermined sequence at the rate of one control word per time slot. In a 128-channel system without subrate switching, the control word sequence required is a 128-word sequence. However, if a group of twenty subrate stations each having a different associated control word is allowed to share one channel of the system, a 2,560-word control word sequence is required for control. Known subrate systems have achieved such long control word sequences by manifold increases in their control word storage capacities.
An additional complication is present in subrate switching systems wherein a first plurality of stations have a data rate of one data word per n channels and a second plurality of stations have a data rate of one data word per m channels where m is less than n and n is not an integer multiple of m. U.S. Pat. No. 4,206,322 issued to J. W. Lurtz on June 3, 1980, discloses a time division switching system directed to this complication. The Lurtz system includes a first storage arrangement having n storage locations and a second storage arrangement having m storage locations. However, in a typical case where many of the Lurtz system channels are not used for subrate data, the required control word storage capacity is still unnecessarily large since, for example, each control word associated with a nonsubrate channel is stored in multiple locations. Further, extension of the system to serve a third plurality of stations having a data rate of one data word per p channels would, for certain values of p, require a third storage arrangement having p storage locations.
U.S. Pat. No. 4,122,309 issued to C. C. Jacobsen on Oct. 24, 1978, teaches a method and apparatus for generating a sequence of signals using first and second read-only memories. The first memory provides for generation of unique portions of the sequence and the second memory provides for generation of a single subsequence which is repeated several times within the sequence. However, typical control word sequences for subrate switching systems are not of the type having a single repeated subsequence.
In view of the foregoing, a recognized problem in the art is that an undesirably large magnitude of complex and costly control word storage facilities are required in time division switching systems for serving multirate data customer stations.